Interposers are used for integrated circuit packaging, typically for routing connections between semiconductor dies and packaging components. For example, semiconductor dies may have tightly-spaced bonding pads, which are inconvenient for packaging processes. Interposers may thus be used for space-transforming, and more particularly, increasing the pitches of the semiconductor dies.
FIG. 1 illustrates a cross-sectional view of an intermediate stage for bonding dies 12 and 14 on interposer wafer 10. Interposer wafer 10 includes copper posts 16 connected to bonding pads 19, which are further bonded to solder balls 20. Dies 12 and 14 are mounted with copper posts 16 through solder bumps 18.
Typically, the bonding process includes pre-mounting solder bumps 18 onto dies 12 and 14, applying flux to copper posts 16, and then bonding dies 12 and 14 onto interposer wafer 10, wherein solder bumps 18 are placed against copper posts 16. After the reflow process for joining solder bumps 18 with copper posts 16, underfill 22 is dispensed into the gaps between dies 12 and 14 and underlying interposer wafer 10. In conventional processes, underfill dispenser 24, which includes needle 26, is used to dispense underfill 22 onto interposer wafer 10 through gap 28 (and gaps between dies 12 and 14 and other neighboring dies). The underfill is then drawn into the gaps between solder bumps 18 by a capillary effect.
The conventional bonding process suffers shortcomings. It is preferred that the width W1 of gap 28 is as small as possible, so that no interposer wafer space is wasted. On the other hand, with the reduction in the width W1 of gap 28, it is hard to dispense underfill through gap 28. The problem further escalates with the down-scaling of dies. Typically, width W1 of gap 28 is required to be greater than about 1 mm.
A further shortcoming in the conventional process is the difficulty in the cleaning process performed after the reflow of solder bumps 18. The undesired substances, such as leftover flux, must be removed. However, with the increasingly smaller width W1 and smaller width W2 between interposer wafer 10 and dies 12 and 14, the cleaning process becomes increasingly more difficult. New bonding processes are thus needed.